After communicating with the Detonics headgunsmith, and making sure he was aware of all the material from this thread, his first response was, "The extractor had everything to do with it". He pointed out that the extractor used in the 451's had a hook that was .020 closer to the breachface than the original Gov't model's hook. (I was pretty sure I'd heard him mention this stuff before, maybe around a campfire on an elk hunt, or something). Anyway, he continued by pointing out that some customers were likely to get the two cartridges mixed up in the same magazine load, and this was one way to reduce the chances of having a problem.

He DOESN'T recommend that anyone make a practice out of shooting 45 ACP's in their 451 magnum barrels!!! "Even though the extractor hook functioned to headspace the ACP round, and did a reasonably good job of it, it's not the best way to get the job done", he remarked.

Hope this helps, ...Deeton